This invention relates generally to radio-frequency aircraft landing systems and, more particularly, to such systems in which an image of an airport runway is generated for the aircraft pilot. Instrument landing systems (ILS) have been in use for many years at major airports, to guide appropriately equipped aircraft along a predetermined course and "glideslope" toward the runway, until a point is reached at which the pilot has the runway in view and can make a normal landing. Under various categories of weather and visibility conditions, the pilot must be able to see the runway before making a final decision whether to continue or abort the landing. Conventional instrument landing systems do not permit a landing to be made in conditions of extremely poor visibility. Moreover, ILS approach systems are relatively costly to install at airports and are, therefore, not available at many smaller airports, even though such airports have to be used regularly by mail carriers and similar aircraft.
Other types of aids have been proposed to facilitate aircraft landings in conditions of poor visibility. In particular, systems using radio-frequency (rf) radiation to form an image of the runway, using radar principles, constitute one category of landing aid. Using a radar-type transmitter and receiver on the aircraft has the significant disadvantage that a transmitted beam must be scanned, either electromechanically with a movable antenna, or electronically using a phased array antenna. Such systems are, therefore, relatively complex to construct and maintain, and do not always provide a satisfactory image of the runway.
Another major type of landing aid using rf radiation involves the use of an rf "camera" on the aircraft, to receive rf radiation from the ground and focus it onto an image plane containing a large number of detectors. The image formed at the image plane can then be used to drive a planar display device. Such a system is disclosed in U.S. Pat. Nos. 4,910,523 and 4,940,986 issued to Huguenin et al. The principal difficulty with devices of the "camera" type is that the resolution of the resultant display is largely dependent on the number of detectors in the image plane, and therefore a large number of such detectors is required to produce a desired image.
Another type of landing aid uses conventional landing instrumentation and generates an image of the runway by predicting where it should be based on readings obtained from various ILS instruments. This might be termed a "predicted" of "indirect" runway image, and is subject to the disadvantage that the predicted image may not be as reliable as one generated from direct rf observation of the runway.
What is needed is an rf landing aid that will generate signals from which an accurate and reliable image of the outline of the runway can be synthesized, for use in poor visibility. The rf landing aid should also generate signals to drive standard aircraft instruments that provide indications of course deviation, glideslope deviation, and range of the aircraft with respect to the runway. Ideally, these functions should be provided with a relatively inexpensive device on the aircraft and with minimal expense for airport installation. The present invention meets these and other requirements.
Another requirement for aircraft taking off or landing is that of safe navigation on taxiways between the runway and airport terminal buildings. Typically, this function is handled by a ground controller, sometimes using ground-based radar, but relying for the most part on visual and radio contact with the aircraft. Smaller airports may not have ground controllers. Many unfortunate accidents on the ground could have been avoided if the aircraft had been fitted with rf devices from which an image of a taxiway could be generated for each pilot. The present invention also satisfies this requirement, as will become apparent from the following summary.